Concentrations of low levels of trace species in gas mixtures, such as, for example, natural gas (methane) may be measured by a variety of different techniques, one of which is absorption spectroscopy. A light beam of suitable wavelength is passed through a cell containing the gas mixture to be analyzed. As light passes through the gas, it is partially absorbed by trace gas molecules. The amount of light absorbed depends on the concentration (partial pressure) of molecules of a species that absorbs light at the incident wavelength. Light intensity transmitted through the cell is proportional to the concentration of absorbing gas in the cell and can therefore be used as a measure of the concentration. This technique is suitable when the background gas has no absorption features in the spectral region being used for the trace gas measurement, but is less useful for complex gas mixtures because multiple compounds in the mixture may have absorption features that overlap with those of the species of interest. In the petrochemical industry, these “interfering” gases tend to be numerous and to have complex absorption spectra, as shown in Table 1, which lists representative concentrations of various species typically found in petrochemical plant vent gases.
TABLE 1Constituent gases typically found in petrochemical plant vents.Component GasPercentagehydrogen sulfide (H2S)0.0085nitrogen (N2)0.0500C1 hydrocarbons2.0667carbon dioxide (CO2)97.7430C2 hydrocarbons0.0721C3 hydrocarbons0.0258IC40.0054NC40.0075IC50.0023NC50.0018neohexane0.0001cyclopentane0.00022-methylpentane0.00063-methylpentane0.0003n-hexane0.0010methylcyclopentane0.0002benzene0.0019cyclohexane0.00092-methylhexane0.00053-methylhexane0.0003trimethylcyclopentaneitoluene0.00402-methylheptane0.00063-methylheptane0.0001dimethylcyclohexanes0.0009n-octane0.0004ethyl benzene0.0004m- & p-xylenes0.0011o-xylenes0.0006C9 naphthenes—C9 parafins0.0008n-nonane0.0001n-decane—undecane plus0.0002Total:100.0000